The genealogies given for St. Joseph in the Biblical gospels of Luke (chapter 3) and Matthew (chapter 1) appear to contradict each other.

According to Matthew his father was Jacob, while Luke gives his father as Heli. According to the early Church father Eusebius, writing about 320 and quoting Julius Africanus writing ca. 200, these can be reconciled by the explanation that Joseph's mother (whose name is unknown) was married twice. Her first husband would have been Heli, who died childless. Under Jewish law of the time his widow could remarry, and Joseph was the biological son of her second husband, Jacob. But since Heli was childless, there were legal provisions that under some circumstances the child of the second husband would be considered the legal child of the first, a form of adoption which has some similarities to the adoption by the dead practiced in the 19th century by the Church of Jesus Christ of Latter-day Saints.

According to Julius Africanus, he had this explanation from members of Joseph's family, and the idea is apparently supported by differences in the wording of the texts: Matthew uses a Greek term which unambiguously refers to biological parenthood, while Luke's phrasing can mean legal as well as genetic parenthood.

As St. Mary the Virgin's husband and Jesus Christ's foster or adoptive father he had a considerable role in the formation of Christianity and thus the course of world history.

This interpretation (assuming Jacob and Heli were brothers) is supported by knowledge of the former custom of levirate marriage among the Jews, where a widow would marry the brother of her dead husband if he had left no male heirs, and the offspring of the second marriage would be considered the children of the dead man. This is in turn very much like the practice of ghost marriage, where any later children of a widow are legally the children of her dead husband. Among a number of cultures practicing this custom are the Nuer of the Sudan, the Swazi of southern Africa and the pre-revolutionary Chinese.

An alternative interpretation is provided, according to Ninan, by the Jewish so-called Law of Zelophehad. Ninan states that this provided for the adoption by a woman's sonless father (even if he were already dead) of her husband. This assumes that Heli was the biological father of Mary the Virgin, rather than her first husband, and that he had no sons. He or his clan would have adopted Joseph, whose birth father was Jacob, as his legal heir. This made Joseph the adoptive, legal son of his father-in-law, Heli, and both genealogies are correct. But Orthodox Jewish sources state that this practice had nothing to do with adoption, but merely provided for the daughters of a man who died without male heirs to inherit, thus ensuring their financial security.

References

Complete Who's Who in the Bible, edited by Paul Gardner. (London: HarperCollins Religious, 1995)
The Interpreter's Dictionary of the Bible: An Illustrated Encyclopedia. (Nashville: Abingdon Press, 1962)
Johansen, Jay. "The Most Famous Adopted Person in History?" Available at: http://www.ohiolife.org/adopt/joseph.htm
Evans-Pritchard, E.E. The Nuer: A Description of the Modes of Livelihood and Political Institutions of a Nilotic People. (Oxford: Clarendon Press, 1940)
African Systems of Kinship and Marriage, edited by A.R. Radcliffe-Brown and Daryll Forde. (London: Oxford University Press for the International African Institute, 1950)
Ninan, M.M. "The Problem of the Genealogy of Jesus. Chapter 3: Luke's Genealogy" Available at: http://www.acns.com/~mm9n/Genealogy/cha1.htm
Schwimmer, Brian(?) "Ancient Hebrew Mythology: A Structuralist Interpretation." Available at: http://www.umanitoba.ca/faculties/arts/anthropology/tutor/case_studies/hebrews/hebrews.html

Indexes

European
Jewish
Near East
Roman Empire
Israel
Palestine
Bce
1st to 10th Centuries Ce
Christian
Jewish
Figures Whose Adoption or Fostering Is Fictitious, Disputed or Unconfirmed
Birth or Infancy
To Provide Heirs, As Protégés, etc.
Customary or Traditional Adoption, Informal and Extra-Legal Care
Parents Married (or Partnered) to Each Other